The objective of the proposed work is to study the composition, structural properties, spatial orientation, and distribution in normal and pathologic platelets of the platelet membrane glycoproteins involved in platelet function. The results will contribute to our understanding of platelet aggregation and adhesion, and may help elucidate the role of platelets in hemostasis and in the development of atherosclerosis. Previous work by others has established that deficiencies of surface-oriented platelet glycoproteins are associated with defective platelet aggregation in the hereditary bleeding disorders thrombasthenia and Bernard-Soulier disease. By using high-resolution electrophoresis, a method which separates individual platelet components, we have shown specific deficiencies of glycoproteins IIb and III are present in thrombasthenic platelets. Our characterization of these two glycoprotein subunits in normal platelets by a combination of the high-resolution separation method and peptide mapping indicates that they contain about 60% homologous peptides. Our cell-surface labeling techniques and subsequent peptide mapping have shown GPIII to be a transmembrane protein, and both IIB and III appear to be exposed mostly on the exterior of the membrane bilayer. The surface exposure of GPIII relative to GPIIb was three- to four-fold less in thrombasthenic platelets than in normal platelets as shown by lactoperoxidase-catalyzed iodination. The focus of the proposed work will be to continue analyses of pathologic platelets by high resolution methods and to pursue biochemical characterization of normal platelet membrane glycoproteins. Considerable effort will go into utilization of the high-resolution two-dimensional electrophoresis method to obtain purified glycoproteins IIb and III.